1. Field of the Invention
This invention relates generally to the field of electrophotographic copying machines, and more particularly to a stripping device used in such copying machines for stripping sheets of copy paper from a heated fusing roller.
Generally, in an electrophotograhic copying machine, an original document is illuminated and an optical image of the document is reflected onto a uniformity charged photoconductive element, usually in the form of a drum or a belt. The photoconductive element becomes discharged in the areas where it is struck by light, which correspond to the non-image areas of the document, but the photoconductive element retains its charge in the areas not struck by light, which correspond to the image areas of the document. Thus, a latent or charge image of the document is produced on the photoconductive member. A suitable colored developing material, or toner, is applied to the photoconductive member and is retained thereon in the charged areas so that the charge image on the photoconductive member is thereby rendered visible, or exposed. A sheet of copy paper is brought into contact with the photoconductive member and the developed image thereon, and the image is transferred by suitable electrostatic techniques to the sheet of copy paper. The toner image must now be permanently affixed to the sheet of copy paper, and one very desirable and frequently used technique for doing this is by hot roll fusing. This technique is desirable because it very effectively fuses the toner to the copy paper and does so without any adverse effect on the condition or appearance of the paper. At least one of the rollers between which the copy paper passes is heated sufficiently so that the toner is raised substantially to its melting temperature which causes the toner to melt enough to be thoroughly absorbed into the body of the paper without smearing. The sheet of copy paper with the fused image thereon is then ejected from the copying machine.
One of the major drawbacks of the hot roll fusing technique mentioned above is that of the copy paper sticking to the heated roll of the fusing apparatus and becoming wrapped around the roll instead of exiting from the copying machine. This occurs because the toner becomes very tacky when it is heated substantially to its melting point for fusing and the tackiness of the toner causes the lead edge of the copy sheet to adhere to the roller rather than passing straight through the pair of rollers to exit from the copying machine. This problem occurs particularly frequently when the toner image on the copy sheet is relatively close to the lead edge of the copy sheet due to the fact that the closer the toner image is to the lead edge of the copy sheet, the greater is the adhesion force of the copy sheet to the heated roll. The above drawback of heated roll fusing devices has been rather effectively overcome in some instances by providing the heated roll with a surface layer consisting of a material having a relatively low adhesion characteristic such as TEFLON, and additionally providing a coating on the surface layer of a suitable release agent such as silicon oil. However, one disadvantage of this solution is the necessity for having apparatus associated with the hot roll fusing assembly for maintaining a supply of the silicon oil and for continuously applying an extremely thin and uniform coating of the oil to the heated roll. Obviously this entails added complexity and cost to the copying machine which it would be most desirable to avoid. Also, it happens occasionally that too much oil is applied to the heated roll and it is transferred to the copy sheet with the result that the copy sheet has an objectionable appearance and texture.
An alternative solution to the problem of copy sheets adhering to the heated roller is the use of a surface layer on the roller consisting of a material which has good characteristics as a release agent without the addition of liquid release agent as or discussed above, such material for example being silicon rubber or fluoro-rubber. However, these materials are not as effective in always assuring that the copy paper will not adhere to the heated roll as when a liquid release agent is used, and accordingly it has become commonplace to utilize a mechanical stripping means in combination with heated rolls which are covered with a surface layer of silicon rubber. A typical form of stripping means comprises a plurality of stripping fingers mounted in close proximity to the surface of the heated roll, each finger having a tapered configuration terminating in a knife edge which lies in contact with the surface of the heated roll to prevent the copy sheet from adhering to the surface and being drawn around the roll.
A major problem with such stripping or separating devices is that they can seriously damage the heated fuser roller in the event of paper jam. Typically, the fuser rollers of a hot roll fuser assembly are confined within a housing for safety purposes, and if a sheet of copy paper is not properly stripped from the heated roller it may press against the underside of the separating devices with considerable force. This force can easily be sufficient to cause the knife edge on the stripper device to gouge into the soft silicon rubber coating of the heated fuser roller and groove the surface to the point of irreparable damage since a groove roller cannot provide uniform fusing of a toner image on a sheet of copy paper.
2. Prior Art
Several attempts have been made in the past to solve these problems, but none has been entirely satisfactory. One such attempt is shown in U.S. Pat. No. 3,844,252, in which a stripper finger is releasably held in a normal operating position by a resilient clip which grips a mounting post. If a paper jam occurs, the resilient clip yields and releases the mounting post and allows the stripper finger to shift to an inoperative position. A principal difficulty with this arrangement is that it requres a considerable amount of force to cause the resilient clip to yield, and damage to a soft roller coating can still result. Also it is difficult to accurately control the amount of force which is required to cause the resilient clip to yield.
Another attempted solution to this problem is shown in U.S. Pat. No. 4,065,120 in which a pair of stripping devices are mounted on a supporting bar which is carried by side frame members of the fusing apparatus. The supporting bar is movably mounted in such a way that if a jam occurs, the stripper fingers are forced into frictional contact with the surface of the fuser roller with sufficient force to cause the mounting bar to move to disengage the knife edge of the stripper finger from the surface of the fuser roller. A significant drawback of this system is that it requires for its operation an increase in the very characteristic which is causing the damage in the first place, specifically pressure of a knife edge on the relatively soft surface of a silicon rubber coating.